Tuesday, November 27, 2007

In the last post we introduced you to Randy, our imaginary 70 kg average male runner, and we created some potential scenarios regarding his fluid and sodium losses and replacement. The biggest take home message was to listen to your body and to drink to thirst, as this has been shown again and again in the field and the lab to keep people from drinking either too little or too much. We have received tons of feedback and discussion, and as we stated in the comments to that post we are pleased that so many of you are participating in the discussion, sharing your stories, and asking relevant and insightful questions.

Just as we will admit that field studies are not the perfect experiment but play an important role, it is the same with prediction equations. We can predict all we want from imaginary scenarios, and some times the equations are pretty accurate, but there is no substitute for the real thing, and many readers wanted to see more actual data and references that demonstrated why sodium ingestion is not necessary, and how both sports drinks and water will can cause a fall in sodium concentration. So while it is important to go through the exercise in the prior post, the next logical step is to look at the actual data.

This study examined ad libitum fluid ingestion in older adults during intermittent exercise in the heat. Basically, they had continual access to water in one trial and Gatorade in another, and they had to cycle for 15 min and then rest for 15 min. The total time of each trial was two hours (four work/rest bouts) followed by an additional 30 min recovery period. A number of variables were measured, but we will focus on the sodium concentrations. One limitation of this study for our purposes here is that it was performed in older adults, and there is a well-documented effect of age on the thirst mechanism so that as you become older you become less sensitive to thirst. That is, your plasma osmolality rises higher before thirst kicks in.

There were sex differences in the fluid intake behavior of older active adults, with women drinking more water per kilogram of BM than men

As we tried to explain in many of our prior posts, the ingestion of any hypotonic fluid in excess of thirst will cause a fall in the sodium concentration. In this case "in excess" means drinking more than to your thirst. This occurs even though sports drinks contain some sodium because they still have much less when compared to the body fluids. Therefore the end result is a fall in sodium concentration. The data from this study show that these older adults, even when drinking to thirst, experience a fall in sodium concentration when ingesting water or Gatorade:

What we see is the time on the x-axis and the sodium concentration on the y-axis. The black dots represent the Gatorade trial, and the white (open) dots represent the water trial. All the subjects started with a sodium concentration of 142 mM per Liter, and in both trials the average concentration fell over time to approximately 139-140. There were no differences between the groups, and the symbols you see on the graph means that those values are significantly (statistically) different from the baseline value. So water and Gatorade ingestion produced a similar effect, and so Gatorade did not prevent a fall in sodium concentration in these subjects.

However one female subject ingested 2.8 L of water and 2.7 L of Gatorade in the respective trials. She gained weight in both instances, indicating an excessive fluid ingestion, and here are the data that support the conclusion that ingesting Gatorade will not prevent hyponatremia:

The problem is that the authors herald this as proof that ingesting Gatorade is much better than water:

"Furthermore, this woman’s data support the notion that a CES [Gatorade] is superior to water in limiting reductions in serum sodium during exercise-heat stress. During the CES trial, this female subject consumed 2.7 L and had a final serum sodium of 131 mmol per L. Therefore, although she consumed similar amounts of CES and water, serum sodium was maintained above that of symptomatic hyponatremia during the CES trial."

While the authors are entitled to their interpretation of the data, we disagree and conclude that both fluids are producing a steady fall in sodium concentration, and that the 131 value in the Gatorade trial is just marginally outside the symptomatic range (< style="font-weight: bold;">ingesting either water or Gatorade produced a nearly identical fall (~2-3 mmol) in the sodium concentration.

The take-home part: Sports drinks do not prevent hyponatremia

In fact Jonathan tried to apply this finding to a more "real world" situation in a letter to the British Journal of Sports Medicine. In that letter he argued that since the mean finishing time of women marathoners in America is five hours, and if the ingestion of Gatorade at rates similar to those found in the study is advocated by races, coaches, scientist, etc., then there would likely be many women (and probably men, too) presenting with hyponatremia. These data demonstrate that sports drinks do not prevent this condition as their ingestion in these subjects and at these rates causes a fall in sodium concentration.

Again one limitation to this study is it was done on older subjects, who are less sensitive to thirst, and what we might see in younger subjects would be a slight rise in sodium concentration when ingesting Gatorade and a maintenance of sodium concentration when ingesting water. The evidence for that statement comes from a 1992 study by Robert Cade, the inventor of Gatorade who incidentally died this week at the age of 80. In that study three groups of runners completed a marathon. One ingested Gatorade, another "half-Gatorade" (50% water, 50% Gatorade), and the third group water:

So in fact ingesting Gatorade to thirst in younger subjects results in a rise in sodium concentration, which is why you drink more---you never lower your osmolality below the thirst threshold and therefore are thirstier when ingesting a sports drink, whereas with water you maintain the osmolality right around the thirst threshold and drink and abstain as your thirst comes and goes. With sports drinks you instead just get thirstier, which seems kind of ironic since their slogan is "The thirst quencher!"

So those are some of the data that support our conclusions, and we hope that helps to clarify some of our interpretation(s) and conclusions. We will still post a "wrap-up" for the series on cramping in which we will try to briefly summarize the main points but more importantly leave you with some practical advice on this complex topic!

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Comments:

The crux of the issue with all sports drinks is that they're hypotonic to body fluids. Most of them that provide some energy are more or less preferable to straight water, if they're tolerated by the athlete, but they don't solve the electrolyte issue in long-duration exercise. A drink that's actually close to being isotonic to body fluids just isn't palatable and no one will drink it simply because it tastes terrible (yes, even when you're low on salt - they taste too concentrated).

The other problem with sports drinks is that people drink them when the extra calories and electrolytes are NOT needed, but of course this isn't a problem for the vendors!

I can't understand Cade's data based on the information you've already given us. Shouldn't the plasma osmolality always decrease from the consumption of a hypotonic fluid?

Maybe I don't understand what you mean by younger people being more sensitive to thirst. I assume it means they feel thirstier and/or thirsty at lower osmolality and thus drink more. The earlier information suggests that, regardless of whether they drink water or Gatorade, their plasma osmolality would then decrease, but that is not what Cade's data shows!

Yes you are correct to think that ingestion of a hypotonic fluid will cause a decrease in osmolality---that is, the concentration will fall.

But what is happening when you exercise is that you sweat, the concentration rises, you get thirsty, and you ingest enough fluid to lower it a bit. . .your thirst goes away, then it rises again since you are sweating, and you drink again, and so and so on.

What I have described above is what happens when you drink to thirst. However when you drink to a schedule, for example 350 mL every 15 min, or when you drink to replace body weight, then you ignore your thirst and continue to drink when it is not necessary.

The result is that the osmolality continues to fall as you are diluting the ECF.

So in fact I would say that drinking to thirst optimizes your fluid ingestion because it helps you drink just the right amount. Why drink 1000 mL per hour when 500 mL per hour is sufficient?

With the bit about younger vs. older, you are actually correct in your thinking. The younger subjects are more sensitive---therefore their thirst mechanism works fine and maintains their osmolality. However in the older subjects things do not work as well, and even though they are drinking ad libitum---they drink when they choose to---their concentration still falls. In fact the data I did not present was their changes in weight, and in the Gatorade trial the average change was positive, meaning they gained weight---about 0.5 kg. In our series on dehydration we spoke about how the normal response to endurance exercise is to lose a little bit of weight. In this case they gained a bit and that is why their sodium concentrations went down.

Apologies as looking back now I should have put that data in, but it seemed to be getting cluttered and I opted to leave it out and rather focus on the sodium concentrations and the Cade data.

So again, in the younger subjects drinking to thirst they maintain their osmolality just fine when drinking water, and they probably lost a bit of weight in doing so. So even in a marathon in Florida they do not need any sodium supplementation to maintain their plasma sodium concentration. Instead, drinking water to thirst does the job just fine.

Excellent question, though, and I hope I was able to clarify it a bit, but please keep asking if not!

Hi Again,I have an interesting hydration/overheating situation to present for your analysis. My brother in law recently returned from a tour in Iraq. I asked him how they could even stay alive in 120 degree heat while wearing full body armor. His response: they drink 2 to 4 liters of ice cold gatorade, EVERY HOUR, to cool the core of their bodies. There has to be a reason they choose Gatorade over plain water. Just wanted to bounce this one off you.

Do we know is alterations in osmalality are worse than changes in plasma volume? These are obviously the two things we are dealing with. An amazing number of factors go into the balance between and among these two parts of the bodily fluid equation.

I think you have very much made the point that drinking to thirst is good. Don't belabor this post anymore because the complexity has consumed it altogether.

The fact is Water and Gatorade will make a person hyponatremic or hypotonic. Anything is bad in excess, so we should just drink to knee jerk rather than brain wave. Case closed.

Now let's hear about hypoxia! I love being hypoxiated, or atleast that's what I call it. Last time I went to my Buddy's house in Boulder Colorado, the first thing we did was drive up into the mountains for a wicked single speed circuit around his favorite mountain bike track. I was all light headed and nausious after about 1/5th the effort I ussually could put out. "Hypoxiated"

Hi energetich----thanks for driving home the point: anything in excess is bad!

That is so, so true in all facets of life, and that goes for fluid ingestion as well. And your other point is important, too: this is a complex issue, and Ross said in a comment earlier the body is not just a jug of water with a bunch of spoons of salt in it. The mechanisms via which we maintain homeostasis are numerous and indeed very complex.

Changes in osmolality are "worse" than changes in plasma volume. But I should say that often one does not get changes in osmolality in the absence of changes in plasma volume. In other words, osmololity normally goes down only when you expand the PV, and vice versa. The only other times you really get a decrease in osmolality is in some pathological conditions such as Addison's disease, which is when a person cannot conserve sodium (due to a deficiency of the hormone aldsoterone) in the kidneys and instead pees it out. This causes a lowered sodium concentration (and thus osmolality).

Great question, and we will do our best to get on with the altitude posts as soon as we can. In the mean time thanks for the support and be careful out there on the mountain!

It seems to me unlikely that Gatorade really helps correct electrolyte imbalances, but people will continue drinking it for two reasons. 1) Liquid calories. 2) Water gets old.The first is helpful in long distance events, where you might eat a bit of food, but can more easily get down a few hundred calories of Gatorade. The second is something many people seem to experience: "water fatigue." That is, they just get tired of drinking large amounts of something tasteless. So Gatorade mixes it up. I suspect these are both also answers to Ryan's question about soldiers sweating in Iraq. For my money, I mostly just drink water or a mixture of water and fruit juice (ad libitum), which solves the two problems above. My body takes care of the rest! Of course, after the workout the best recovery liquid is beer. (Please don't write a post disproving this!)

Hi William, and thanks again for participating here in this discussion on The Science of Sport.

Great point about switching between the two drinks. In fact in our field studies at the Two Oceans and Comrades Marathons, this is exactly what we find. Nearly all the runners have a pre-planned strategy that involves drinking sports drink at one station, then water at the next, and so on an so forth.

Their plans are based on their experience like yours---they know that after two hours of drinking the same thing, it just becomes totally unpalatable.

Also, as you have stated sports drinks most certainly have a role to play, and that is because of the CHO content. For the marathon or longer you have to ingest CHO, and when especially running it is much easier to swig a few hundred mL's of fluid then to eat an energy bar or gel, and of course if you are on solids then you have to carry them. . .not too practical for a marathon.

Cycling is a different story, and I rely strictly on water plus gummi bears. Gummis have two excellent ratios going for them:

Grams CHO/Total MassGrams CHO/Unit of cost!

Plus I love sweets and they are easy to swallow (or inhale if you go over a bump while chowing!). I found early on that sports drinks just make me thirstier and thirstier over time.

I think we will have to do a post that proves beer is an effective recovery drink! Another anecdote from Oceans/Comrades is that a lot of the runners I have chatted to prefer beer after the race because they have just been drinking syrupy drinks and gels for hours on end, and the last thing they want is something else sweet. Therefore beer seems to hit the spot just fine!

Hi Ross and Jonathan. My data agree on sports drinks but disagree on fatigue as the rigor complex cause. A simple experiment will show why.

Take two athletes with EQUAL finish times where neither is given electrolytes - a cramper and a non cramper. Then have them race where you give them BOTH electrolytes. The cramper will finish SIGNIFICANTLY earlier. WHY??? Because he has to be fitter to compensate for the cramping and still finish at the same time, when he Doesnt have electrolytes! If cramping were simply fatigue then electrolyte replacement would have the same effect on both athletes. But it doesnt!

Also if fatigue were the cause, fitness and training or alternatively racing slower would by the remedy. But a real cramper will cramp at the same mileage even if he gets there slower.

My theory is that crampers are NOT salty sweaters but high volume sweaters. They see the salt accumulation on the skin and assume its high salt concentration, when its just that more sweat passed through the same area. But perhaps they sweat twice the volume at normal concentration and attempt to replace by drinking, because they are thirstier. But their absorbtion is SLOWER than sweat rate. So they stay thirsty and even drink more! When we measure their body weight at the end we assume they just over hydrated but we were deceived into thinking ALL the water consumed was absorbed!!!!... They cramp because they have induced a hypotonic state by over drinking...(make sense?)... this would also explain why supplements help them but NOT non-crampers.

Some literature suggests that there may be a role for "pre-event" sodium supplementation to increase plasma volume to improve heat resistance and aid performance. I imagine this would be something like using glycerol as a plasma volume expander.

All the subjects started with a sodium concentration of 142 mM per Liter

As a biochemist, I'm a little confused. You mean 142 mmol/L, right?

Anyway, in my line of work, a difference in concentration of even 1 mM within the context of the cellular environment is huge. I'm not sure what exercise physiologists think of this difference. Perhaps you can enlighten me.

Anyway, from these data, it seems that drinking any hypotonic solution will lead to hyponatremia. I think the point being made by the authors is that you can stave it off longer with consumption of CES. which is reiterated in subsequent comments to the paper. I'm not sure why you guys disagree.

It's funny, since the trend appears to only be exacerbated in females, and they only show data for the crazy 2.4L trial with water and CES for a single female. It would have been nice to have seen data for a male subject.

If you see no difference in a male subject using CES or water (assuming a 1-2 mM difference is negligible), that says something.

Yes, we do mean 142 mmol/L. You are correct that even small changes in the concentrations on either side of the cells will create fluid shifts, and this is exactly why our thirst mechanism works like it does. That is, our bodies defend the osmolality and not body weight changes.

So yes, small changes are important, and when people drink to thirst they maintain this osmolality, and therefore they protect the fluid "compartment" volumes (the ICF and the ECF).

As the same time I will say that a fall of one mmol/L will not cause such large fluid shifts to produce encephalopathy. But the point is that the body tries to defend that osmolality to prevent fluid shifts.

Re ingesting hypotonic fluids. . .yes, again you are correct that the ingestion of these fluids will indeed cause a fall in osmolality when ingested in excess of thirst. When you ingest water or even Gatorade to thirst you do not get a fall in plasma sodium concentration, and it is the excess ingestion---beyond those volumes associated with thirst---that cause problems.

We disagree with the authors interpretation because we feel the more important finding is that ingesting so much fluid that you gain weight is a bad thing regardless of the fluid. We felt their conclusion was not strong enough in that the primary emhpasis should be that this woman drank way too much fluid. Then, only as a distant secondary point, can one say that Gatorade prevents a lesser fall.

But again, it is still falling, and that goes entirely against what the body is trying to do. Therefore the simple solution is to drink to thirst to prevent this.

Hyponatremia has a much higher incidence in women than men, and no one has really been able to hypothesize anything beyond the mass issue. In that study apparently none of the men gained so much weight as that one woman. However, if you look at the first figure we see that when these elderly subjects ingest enough fluid to maintain their weight (in fact they gained just a little bit) their osmolalities fall, and that can be problematic. (NB: we did not show the figure with the weight changes in the post).

However since this was a Gatorade study, they will conclude that Gatorade is the superior beverage compared to water, and in fact they have ignored the finding that 1) sodium concentrations are falling in both conditions, and 2) that is not what is meant to happen during exercise.

Great questions and points, though! We are glad to have you on board and hope to see you in more of the discussions.

“So in fact ingesting Gatorade to thirst in younger subjects results in a rise in sodium concentration, which is why you drink more---you never lower your osmolality below the thirst threshold and therefore are thirstier when ingesting a sports drink, whereas with water you maintain the osmolality right around the thirst threshold and drink and abstain as your thirst comes and goes. With sports drinks you instead just get thirstier, which seems kind of ironic since their slogan is "The thirst quencher!"

Hi, I love your blog; I am an exercise physiologist myself and fully enjoy reading your interpretation of data. Since I am a new reader of your blog, I have been reading your series on fatigue with great interest. You make an excellent point in drinking to thirst. For the majority of people completing shorter run races or sprint triathlons, drinking to thirst is an excellent way to go. I often remind my own clients in the days before a race, not to overly drink water or fluids to hydrate- they will in almost all circumstances be able to do it according to thirst level. My comment mainly has to do with the quoted section above. As an ironman triathlete myself for 10 years and coach to many, drinking to thirst with water only does not work for long duration endurance events at this level. Fluid replacement drinks serve to replace the necessary amount of carbohydrate to fuel exercise over a long duration. By increasing the sodium concentration of the drink, it does encourage the athlete to drink more and by scheduled drinking on say, 15 minute intervals; the athlete gets a few swigs in so that hopefully at least 15-16 oz are taken in over the hour. I have found that if an athlete just drinks when they are thirsty; they sometimes forget to drink at all which can have disastrous effects later on in the competition, especially when heat is an issue. For this reason, the little bit of sodium and palatability of the drink helps to keep the athlete in a stable state. Over the past couple of years, Ironman has been sponsored by “Gatorade Endurance Formula” which has an even higher amount of sodium! I was always leery of this product especially because essentially drinking salt water serves to increase dehydration as salt diffuses across the cell membranes forcing water out into the interstitial fluid which I also think contributes to the increase in athletes feeling bloated during the last 20 miles of the Ironman bike leg. I am curious as to your thoughts on this product. It does not work for me personally and many of my athlete friends and clients have switched to completely carrying our own fluid replacement. Exercise nutrition is extremely personal and finding what works definitely takes some trial and error. Keeping in mind the increased sodium level of the endurance formula, many athletes have complained about being really thirsty on the run portion of the half ironman or ironman distance and craving water! For this reason, most of the people I work with ingest Hammer nutrition products on the bike and complete the run on water and gel every 4-5 miles. It goes a long way towards decreasing the gastric distress associated with too much Gatorade and gel on the bike. No doubt from the increased osmolality of the fluid and gel combo not being absorbed.In any case, your blog is great- keep on!jg

Your point is well taken about the need for energy ingestion during endurance events. I think in that series we were focusing only on the fluid aspect, although we might have mentioned CHO ingestion somewhere in there.

But again, CHO/energy ingestion is crucial for any event lasting more than two hours, and sports drinks are a great way to get some of this energy.

However, I would say that one does not forget to drink, and in fact the absence of drinking would indicate that the athlete is not (that) thirsty.

I will go further to say that since the body is trying to protect the plasma osmolality, it is unlikely that our fluid requirements remain constant over the course of an event such as the Ironman.

With regards to the product you mention, it is only a gimmick by Gatorade. Even ingesting Gatorade to thirst during a marathon will increase the plasma osmolality (whereas drinking water maintains it), and so ingesting anything with more sodium than Gatorade will just increase it still more.

This in turn will likely lead to higher thirst sensations and so I am not surprised that you know athletes who crave water after ingesting the Gatorade formula.

Finally, your point regarding the personal aspect of sports nutrition is a good one. The science must serve as a guide and a starting point, but people and coaches must be willing to tweak their practices if it is not working for them.

Thanks again for commenting, and I hope you enjoy the rest of the series.

The idea that sodium ingestion is required is quickly becoming a problem and seen as the only way to prevent hyponatremia.

The primary problem with the study mentioned in that article is that they refer to proper hydration as replacing 100% of one's weight losses. In fact this is incorrect and drinking to thirst is the the only appropriate hydration strategy.

What they found makes perfect sense based on their methods. Since the cyclists replaced 100% of their weight losses, they were replacing more than was required by their bodies. . .hence the sodium concentration decreased. However I would argue that if any of those subjects ingested water to thirst, they would maintain their sodium concentration within a very narrow range, perhaps 1-3 mmol per Liter, and the concentration likely would increase slightly.

That they showed sodium concentration was higher in the sodium group again makes perfect sense as ingesting water to replace 100% of one's weight losses would definitely cause sodium concentration to fall, and therefore any sodium ingestion would attenuate that fall.

To repeat, however, if those athlete drank to thirst, their sodium concentration would likely be maintained just fine.

The funding for that research was not disclosed, but among Exercise Physiologists it is a well known fact that the lab at UT Austin has received steady funding from Gatorade over the years.

So thanks again for the link, it is something we will probably revisit as summer is in full swing and the idea that sodium replacement is the way to go is becoming more and more pervasive, even though it is a bogus concept.

Yes, I set that up on a tee for you to smack out of the ballpark, didn't I?

So even "salty sweaters," among whom I'd be considered by the definitions I've read, maintain the right balance of sodium by drinking to thirst -- since that is practically the whole point of the thirst "mechanism" during exercise. Correct?

You are correct in your conclusion, i.e. drinking to thirst will sort you out.

One additional point to make is that, although measuring sweat sodium is problematic and not necessarily 100% reliable or valid, there will be plenty of individual variation of sweat sodium. Some of the reasons are due to acclimatization and training status, but even if we controlled for all of these things I suspect we would still have individual variation---that is just normal physiology.

So some of us will most certainly lost more sodium via the sweat, but as long as you are drinking to thirst then your body will replace what it needs to. This is one of the major points about this argument. Drinking too little (below thirst) will affect performance, while drinking too much is dangerous (hyponatremia), but drinking to thirst prevents both of these scenarios.

We all have individual fluid needs, and drinking to thirst will ensure you meet your own needs, regardless of the race distance, intensity, or environmental conditions.

About a month ago I read the series on Exercise and Dehydration and found it very interesting. Today, I found a reference to a new article and would like to know your take on it. It seems that athletes performed better while swishing a sport drink than when drinking it (same goes for placebo, better performance for swishing than for drinking).

I have followed and applied Schwellnus' theory on cramps, and Noakes' theories on fatigue and dehydration/water intake for about a decade now and I must say that they have always worked for me and my athletes.

However, recently I have been contemplating a question that I am hoping you could shed light on.

It is common knowledge that the timing of nutrients (particularly CHO and Protein) is important for decreasing catabolic and increasing anabolic reactions (ie it's important for recovery). Usually it is recommended that one should consume a certain ratio within 20 to 30 mins post activity.

Often, during this period, one does not feel like consuming anything. But we do so because we know it is beneficial.

Is it possible that the same could be said about water? Could there be a window of opportunity that might prove to be beneficial for recovery? Could there, in fact, be an advantaget to conuming water, if one isn't thirsty (ie doesn't feel like it)?

Gentlemen,Thank you for your blog! I am grateful to have the mind opening... Although I think I am 'buying' much of what you argue, I have some thoughts and questions for you. I'm serving as 'devils advocate' on some thoughts - though I'm not gator-dupe.

Do like Cats and Dogs?Drink to thirst. Makes sense. Nice and simple. It should and does work most all of the time. But I have to say that animals sometimes do need encouragement to eat or drink when they are sick...that is the first sign sometimes when something isn’t functioning properly. Also, our dog and cat don't typically run 50 miles in the heat in a few hours time just for the experience... and they don't bike 200 miles in a day just for the accomplishment. Not to mention - dogs don't sweat!!! Nope - on hot days, our dog sits in the shade and pants...no exercise for her. So, comparing our physiology and endurance exercise requirements to the thirst obeying needs of "cats and dogs" is about as useful as comparing "apples with oranges".

Why should we be any different from other animals, in terms of ability to eat or drink? Well, we didn't evolve mechanisms like the kangroo rat or the eland or the oryx or the camel. Instead, we've got a lot of our selective advantage due to our brains - for better or for worse! Obviously overhydrating and dealing with belly slosh and running the risk of hyponatremia is due to 'blindly' drinking too much water (or gatorade or whatever). But isn't it in the heat of the challenge that we make poor decisions - such as going out too hard or going with a break versus staying in the pack....or drinking too much or too little? Certainly I don't feel like I'm all together 'there' mentally after an all-out effort for 4 hours in 95 degrees F...even with drinking to my thirst. So I question the call to just 'listen to your thirst'. Common sense aint so common. A lot of physical signs (such as pain) can be overlooked during competition. Are you so sure that thirst will never be one of the symptoms ignored or forgotten?

Is thirst enough?I've read some reports (IOC manual on Nutrition in Sport) that dinking a small amount decreases thirst before dehydration is corrected. I've also read in your posts that thirst “comes and goes” again and again. These bits suggests that thirst isn't sufficient to avoid dehydration.

Isn't someone with exercise performance and cognitive performance deficits due to 5% dehydration potentially 'sick' to the extent that their bodies aren't functioning ideally or properly? So, isn't it then possible that 'drink to thirst' won't always work, and it may be wise to do body weight checks in events such as the Western States 100?

So, isn't it possible that a human running or cycling in 90+ F heat for many hours could become less than healthy and less than able to adequately hydrate based only on thirst mechanism? In the mention of the soldiers in the desert, there was one in the ad libitum drinking group who failed to complete the distance (denominator not given)....

The fact is that many of our homeostatic mechanisms DO fail from time to time. You state that 3-5% body weight loss is normal and to be expected. How do you contend with the many reports of performance deficits beginning at 2% bw loss? Is that a hoax too, or do you suggest performance deficits are just something to put up with and not be concerned with? Do you contend that the performance deficits shown with 2 - 5% bw loss are conclusions made from "over-interpreted" studies? Certainly you don't think that 5 - 7+% body weight loss is no big deal?

Perhaps my questions/thoughts demonstrate that I am just trying to cling on to the dominant paradigm... but I'd love for you two to take this a little further.

hey guys, can you comment on whether or not differences emerge in the strategy one should take re: hydration/electrolyte ingestion when you participate in ultra endurance events such as ironman triathlons?

do we ever run out of or need to supplement with sodium or other electrolytes?

this article had me wondering:http://www.slowtwitch.com/Training/General_Physiology/The_Math_of_salt_loss_1093.html

I know these posts were from 2007, but I was recently directed to your hydration series. I read, with great interest, all of the posts, and am slowly implementing your ideas. However, I continue to have a conundrum concerning salt, fluid and calorie intake. I perused your blog for more information regarding these three entities and how they interact, but didn't come up with anything of significance. So, I'm here to ask... how does the salt concentration relate to calorie absorption?? My coach tells me to take salt, but not for hydration purposes, but for sugar absorption and to prevent nausea. Can you guys elaborate on this subject? I would be much obliged.

your sugars with the exception of fructose are absorbed in the intestine via sodium-sugar cotransporters. Therefore the salt your coach is telling you to take contributes to the available salt in your intestine that is necessary for the absorption of your carbs. I don't know why salt helps with the nausea.

Some literature suggests that there may be a role for the event "before" sodium supplements to increase plasma size to improve performance and heat resistance. I imagine this would be something like using Glycerin and expanding plasma volume. Any insight add electrolyte in this area? Cheers

Jonathan Dugas, PhDCurrent residence:Chicago, USAEmployment: Director of Clinical Development, The Vitality GroupResearch interests: Temperature regulation and exercise performance, with a special emphasis on how fluid ingestion affects those two things. In addition, the effects of exercise on health improvement and risk modification in large populationsSports interests: Cycling, running, triathlon, endurance sports

Full discolusre:The views expressed on this site are not those of the University of Cape Town (UCT), the Sports Science Institute of SA (SSISA), The Vitality Group, or Discovery Holdings.